Developed countries have stringent requirement on the power quality. Power quality issues result in huge economic losses for the industries, such as operation shutdown and increased costs for restarting, damage to sensitive equipment, product rejection, and reduction in product quality, which lead to sale difficulty, damage of company image and commercial relations with customers. In addition, they damage equipment of important users, such as medical institutions, resulting in serious operational accidents. A report by the Electric Power Research Institute in the United States shows that power quality issues cause loss up to 30 billion U.S. dollars in data, materials, and productivity to American industries each year (Electric Power Research Institute, 1999). Other developed countries, such as Japan, also have high standard for the power quality. With quick development of the high-tech industries, China is also imposing higher requirement for the power quality. The voltage sag (or collapse, dips) is a major issue that jeopardizes the security of power-using equipment in addition to causing voltage quality issues. Failure of power systems, starting-up of large generators, and short-circuit of a branch circuit all contribute to the voltage sag. The time of the voltage sag is transient, but it causes disruption and shutdown of industrial operation, and the disruption and shutdown time far exceeds the time of the voltage sag, therefore, the damage is huge.
Traditional methods such as voltage adjustors do not solve the problem. Although uninterruptible power supply (UPS) devices do solve the problem, the cost and operational expenditure are high. To solve the problem, research has been carried out both in China and abroad on dynamic voltage compensators. Compared with the UPS, dynamic voltage compensators effectively solve the problem of the voltage sag. However, energy storage still remains a plaguing issue for dynamic voltage compensators. Although advanced methods, such as minimum energy injection, have been proposed, extra energy storage has been affecting their popularization and development.